1. Field of the Invention
The invention relates generally to the field of subsurface exploration. More particularly, the invention relates to techniques in which instruments equipped with antenna systems having cross-magnetic dipoles are used for improved subsurface electromagnetic measurements and imaging.
2. Background Art
Electromagnetic (EM) induction and propagation logging techniques are well known in the field of hydrocarbon exploration and production. A typical EM logging tool comprises a transmitter antenna and one or more (typically a pair) receiver antennas disposed at a distance from the transmitter antenna along the axis of the tool. The tools are disposed within a subsurface formation that has been penetrated by a borehole to measure the electrical conductivity (or its inverse, resistivity) of the formation. EM energy emitted from the transmitter interacts with the borehole fluid (“mud”) and surrounding formation to produce signals that are then detected and measured by the receiver(s). By processing the detected signal data using inversion algorithms and models well known in the art, a profile of the borehole or formation properties is obtained.
Geologists and petrophysicists historically have found it necessary to visually analyze full well cores extracted from zones of interest to assess complex or thinly laminated (also referred to as bedded) reservoirs and aid in the discovery of hydrocarbons. High resolution “microresistivity” measurement techniques have been developed over the years to contribute to the identification of hydrocarbons in low resistivity pay zones. High resolution measurements have helped improve the estimation of reserves in such reservoirs.
Microresistivity tools have been developed for wireline and while-drilling applications. Examples of wireline microresistivity tools include the Formation MicroScanner™ tool and the Fullbore Formation Microimager tool (FMI™) produced by Schlumberger. Logging-while-drilling (LWD) EM tools capable of providing subsurface images are described in U.S. Pat. No. 5,235,285. The '285 patent describes an LWD tool that can measure the resistivity at the bit. Examples of tools based on this and related principles include the RAB™ (resistivity at the bit) and GVR™ (geovision resistivity) tools produced by Schlumberger. These tools are capable of providing borehole resistivity images of the reservoir rock being drilled.
Early microresistivity techniques were implemented for use with conductive muds, usually a mixture of salt water and weighting solids to control mud density. These EM tools have been designed to investigate the formation beyond the invaded zone present when the well is drilled with a water-base mud. More and more new wells are now being drilled with oil-based mud (OBM) containing chemical additives that build and leave a thin impermeable mudcake and usually prevent significant invasion into the permeable zones around the borehole. Synthetic-based muds have also been introduced in the industry. Early microresistivty tools were based on low frequency electrode devices that are not very suitable for wells drilled with OBM.
Conventional logging tools have been developed to provide subsurface images in wells drilled with OBM. U.S. Pat. Nos. 3,973,181, 6,191,588, and 6,600,321 describe tools capable of imaging operations in OBM. While progress has been made in the development of wireline OBM tools, the development of EM tools suitable for while-drilling operations in OBM has been relatively slower. A need remains for improved EM imaging and logging techniques, particularly in while-drilling applications using OBMs.